High-temperature box annealing process for ferrous vitreous enameling sheet stock



July 15, 1952 ECKEL 2,603,577

HIGH-TEMPERATURE BOX ANNEALING PROCESS FOR FERROUS VITREOUS ENAMELING SHEET STOCK Filed Jan. 12. 1949 2 SHEETS-SHEET 2 TYPICAL coNv NTIoNAL BOX ANNEALING TEMPERATURES FOR ENAMELING sI-IEET STOCK I400 I200 *SOAKING 1 a, PERIDD I R TIME AT g lOOO,-- TEMPERATURE TEMPERATURE READING AT ToP oUTER EDGE DF 1 sI-IEETs 0R COILS If TEMPERATURE READING 2 60 AT CENTER OF BOTTOM COIL DR PII E OF sI-IEETs ooIIIIIIIIII' 5 I0 I5 as so as so I-IoURs IN FURNACE 13g. 4.

TYPICAL CONVENTIONAL NORMALIZING cURvE FOR ENAMELING sI-IEET STOCK LL I700 i l.|.l I; APPROXIMATE DIscI-IARGE TEM ERATURE 5 Q I000 T- z Lu lo I I I I I I I I I I I I I ,i I

o I 2 3 4 5 s 7 a 9 I0 II l2 l3 l4 l5 TIME IN MINUTEs INVENTOR JOSEPH C. EGKEL 4% ,dmwaw 45 Patented July 15, 1952" UNITED STATES PATENT I OFFICE 2,603,577 I y it HIGH-TEMPERATURE Box ANNEAIJING PROCESS FOR FERROUS vrrREoUs ENAMELING SHEET s'roo Joseph C. Eckel, Pittsburgh, Pa., assignor to v United States Steel Company, a corporation of Application January 12, 1949, Serial No. 70,514 v This invention relates to the quicker, cheaper and moreefiicient quantity production of iron or steel sheet stock suitable for vitreous enameling. More particularly, it involves the novel application of but a single and unconventional box annealing operation on such enameling sheet "stock in loosely coiled strip form at temperatures within the critical range of temperature for such stock. p ,v V V "It is common in vitreous enameling practice to-use'as the enamellng sheetstockasi'ngle car'- 'bon steel or so-called-lowmetalloid enameling iron or steel having a very low carbon content, usually not in excess of from .04% to 0.05% carbon; In one particular practice, the chemical composition .oi a' representativerimmed' or opensteel used for this purpose might contain, by way of illustration and not limitation to the recited speciflc'proportions: -carbon .03%; manganese--:10%;' phosphorus .015%; sulphur 2030%,' silicon.010%; residual amounts of copper, nitrogen and chromium; with the balance being substantially all iron.

Such steel is frequently made by rolling it into slab form, then hot-rolling the slab on a continuous hot strip mill, pickling the strip to clean and descale it, and cold reducing the strip on a continuous or reversing cold reduction mill to the ordered thickness, if the ordered thickness were below that conveniently attainable by hotworking. The material may or may not be cleaned prior to heat-treating. In one manufacturing process, the steel-was then shearedto length and normalized at about 1700" F. sheet temperature, and cooled toabout 1000 F. before being air-cooled substantially to atmospheric temperature and pickled. Such sheets had good vitreousenameling characteristics but if additional" deep-drawing properties were required, they were then box annealed'in the range around ir'o m'about' 110o'F, to about 1250 Elm improve their' d'uctility by relieving any residual stresses resulting from the normalizing treatment. And in some "cases, the conventional b'ox annealing step-preceded the" normalizing.

Normalizing operations by virtue of the larger investment and greater handling add materially to the cost of producing vitreous enameling sheets. (Imus, a single or a very limited number of sheets 'canbe-treated at one time in normalizing sothat the output tonnage per hour" of sheets is tremen dously curtailed and the unit cost thereof corre pondin '1 incrasea- Moreover, the air-coolin; pf the "normalized sheets scales them, necessitatirig subsequent I additionalhandling' and ex 4 Claims. (01. 148-12) pense for further pickling treatment. In addig tion, the-relatively quick cooling achieved byaircooling adversely affects the softness of' the sheets, requiring for some applications the further box annealing step which has been men.- tioned. Although the coolingoi -a conventional normalizing operation is faster. than thercooling of 'aconventional box annealing operation, :this

maximumsheet temperature of aboutl26Q-:,F!.

Since such box. annealing. is usually conducted in a neutral or a deoxidizing atmospherefsuch as a natural gas atmosphere, no subsequent pickling or other cleaning is usually 'requiredgor is necessary.- For special applications, an oxide coatingis'sometime's produced by' the use ofa controlled oxidizing atmosphere during anneal-- ing. Moreover, the tonnage output per hour for box annealing is large by comparison with normalizingbyreason of the greater weight of metal that can be treated in a conventional box anneal? ing operation in a given time. i ;r; However, such subcritical temperaturebox'annealed sheets do not exhibit "as a general ruleta grain structure characteristic suitable for deep drawing purposes and hence are-confinedin their application to limited uses such as simple table tops andother relatively'flat-panel applications.

This is so whether the sheets-are sold 'i11='3.011t lengths or in coiled strip -form, withor:w ithout temper rolling, and even though the usual slow cooling is employed. "Nor-are: the mechanical properties of suchsheets 's o-fbox" annealed as good as those of sheets subjected'to normalizing alone or to both normalizing and box annealing.

-Heretofore' in the art ofbox annealingfer'rous vitreous enameling sheet stock, the temperature of such annealing in theen'ameli'ng sheet art has been carefully kept below the critical range. One reason for this practice was that it hadbeen con sidered that increasing the temperature of'su'ch box annealing favored an objectionable nonuniform grain structure whichfif not restricted, upon drawin'g resulted in'=the'-mat'erial pulling coarse." When noticeable to the-nakedeye, it:is V

known as the orange peel" effect and destroys the commercial value of deeply drawn sheets intended for finished smooth surface applications. Moreover, when sheets instead of loosely coiled strip are box annealed above about 1300 F. the sticking ,-.or welding :tendency :of the sheets increases to an extent which makes "such box annealing of stacked sheets impracticableif ton.- na'ge production and acceptable product yield are to be maintained. If such sticking occurs, the

sheets or strip may have to lbeire'je'cted orthey' may tear in subsequent handing. 1

Box annealing in such subcritical temperature ranges as a rule does not eifect -=complete irecrystallization of the grainstructure'o'f the sheet or strip. However, it did stress relieve the rain structure by producing su'fiicient :reformation:

thereof to provide suitablesoftness and ductility in the finished sheet or strip for very limitedapplications. Such conventional box annealing below the critical temperature-also:reli'eved :strains 'prdduced b'y prior cold wvorking 'or any prior nor- 'maliz'ing' 'of-the metal. J 1 j fi'side from @the normal metallurgical :rea'sons, where was another and possibly .more substantial one-"in the'yitreous-enameling sheetsstock art :for maintaining the maximumtsheet' temperature :at

about l 2' 60 F. in ithe b'ox annealing operation.

It had b'een determined 'itllatl unstable carbon: 'or carbide formations :resulted rat vhigher rbox annealing temperatures which moved into the critifoal lte'mper'ature mange. Without theioilowing belief being intended to limit the scope of this invention in any way itzzappearsathat :such car- "hides --*te'nd to 'ragglomerate and form .unstable "iros'ettesflin :tire sheets or .strips :vwhen -'box annealedfiromrbetweemabbut:lzfiflii t toi300Etgas ailowerliimit-"andz'azbout;ia&5058'; aszan unpenlimit. Subsequently, 'iupon 1the .firing :of "vitreous -i enemanat s xorlzsuch' sheets at temperatures approximatingsahoutizfifioir.,zzmirror'designblisters nr fmirtain'si :nopperheads overlyingsuch 4 invention will appear from the drawings, which are illustrative only, in which 7 Figure 1 is a vertical cross section through a gas-fired radiant tube annealing box;

Figure 2 is a section taken along line II-II of' FigureLiI; v

- FigureJB is 1 a 'diagramshowing:typical annealing temperatures in conventional sheet or coil ibox annealing for enameling' sheet stock;

.venti'onaknormalizing curve for enameling sheet stock;

Figure 4 is adiagram showing a typical con- Figure '5 'is' a diagrammatic view of a portion of the iron-carbon; phase diagram illustrating the temperature ranges employed in prior ferrous enamelin'g- 'sheet production practices;

Figure 6flis a..diagrammatic view of a portion of the iron-carbon phase diagram illustrating the criticaltemperature range employed in this in- "frosettes tooffrequently T551111; .in the enamel 'ivlides 'aprocess inwhich a*singleunconventional box rami'ealingoperation is employed :for .making vitreous :enamelingksheets for strips :having good :iiring, *drawing and mechanical -.characteristics anii properties In addition, the grain structure zproduced oy the practice of this invention not i'onlyzhasrgoodrdrawing ,properties but it is sub rstantially' uniform in icharacter. This invention eliminates the needl'for normalizing or for com-'- ibining': offa normalizing and conventional'boxanmealingzinthe commercial production of suchenwamelssheet stock. The-carbon or carbides'in the rsheetsproduced in accordance with this invention are'either'so dispersed'or so stabilizedas to ieliminategprevious rejections because of such blisters and copperheads. Hence, the cost of producing suitable ferrous enameling sheets is :hereby markedly reduced-and the art .materially i-advanced. Other obj ects "and advantages of this vention;

.Figure 7515 an enlarged representativezpictorial view of vunstablev rosettes/ etched j: to :bring but the carbide-structure :a'ndgrairr boundaries Figure 8 is an enlarged representativegpictorial view -:of the material shown zill Figure 7. etched to bring out "the carbide-structure only and not the grainzboundaries sand Figure-Sis an-enlarged representativefpictorial view of a section of a sheet made in accordance with this-invention in'whi'ch ethe carbides are: in

substantially :stableand-dispersed form, said sheet being etched xtogbring out thescarbon elements only. a I Referring to Figures 1, and 2;=a= commonzpractice; in conventional box-annealing involves ithe use; in vone ztypenf :annealing =furna"ce, .01? :a socalled radiant tube-ianealing ibox. :Such a box comprises asheavy castmetal base Hi usuallyzcovered :by .a :low platform :of refractory brick M which platform "ends short ;of :the: outerfledgeziof .base "I'll; .A -ra-d'rant 'tube annealing Ibex; -l-2 or metal lined with refractory ibrick l3 and. having an arched refractory top ,:portio'n lfifisfsupported byibase :lil'when closed-and :fitsover; platform H during the :annealingoperation. The annealing charge may or may not :be covered by a retort-or inner cover. ,tBetween operations, box I12 :is'lifted out of the way by a crane engaging eyes in ears |-2'--on1-box 42. Heat is supplied to the interior of the annealing abox' bybanks of radiant tubes l5 within which natural :gasor-other suitable fuel is -.burned-causing the tubes :to become :incandescent. A deoxidizing atmosphera such'as natural gas, is preferably recirculated through theinteri'or' of box l2 by means of Pine connections l6, :and maintained at-a pressure slightly greater than atmospheric sothat air will nothave access to the .said interior thereof. -:And-sometimes -no outside gas is'circulated through the annealing box. A-gas seal 515 provided -when-box I2 is closed by a sandorgp'owdered'refractory ring 16" around the base, of box 12. In the described reduced ferrous enameling; strip stand on edge singly or in stacks on platform Hx=t prevent sticking,

r r In the prior practice, suchcoils are heated "to a so-called maximum sheettemperature'ofabout 1250" F. -In so doing, allowance has to be made for the-temperature differentialof possibly '75 F. or higher between the top outer edges and bottom centers of the sheet or coilsy a differential which exists in all box annealingfurnaces as illustrated in 'Figure 3; As mentioned earlier,

.the stackingof sheets imposes temperature tlimitationsdue to the tendency to stick which "are not p es'ent when loosely 'coiled strips are annealed, in accordance with the preferred con ventional practice in the art. In some 'cases,;the sheetsjfcanbe stacked vertically and, spaced to avoid sticking but such a methodis not practical by' ciomparison with annealing loose coils. However, even when conventionally annealing loose coils) or spaced sheets the problem of the poor grain structure for drawing remains. As shown in Fi ure. 3, such coils are generally held atanhealing temperature for from about to hours to insure obtaining as much of the softness and ductility required. and appropriate grain as possible, in the" finished sheet. whethensold in cut lengths or in the form of coiled, strip, had good enameling properties but poor drawing and mechanical properties as mentioned earlier.

On the other hand, when such sheets are sub-'- jected, as illustratedin Figure 4, to the more costlylnormalizing in the temperature range extending upward from the A3 point which for steel compositionsof the type with which this inventiondeals varies upwardlyfrom about 1625 F. for. a period of time at temperature varying from about '.1.to 4 minutes, the entire grain structure is recrystallized and made uniform. Sucha normalized product also has good enameling proper ties, and a somewhat improved drawing property insofar as grain structure is concerned. However,

the relatively rapid air-cooling to which normalsents the upper critical or A3 limit. A shaded line IBa indicates that part of the conventional line ofphase change in the iron-carbon diagram extending from line l8 at about 0.035% carbon substantially to the A3 line IS. The shading is employedinorderto portray in an approximate manner the spread applicable in the precise locationof these critical limits for metals of different composition and for the respective heating and cooling operations in that range for a given steel. The intermediate A2 limit where the alpha iron changes to the beta form is illustrated by a line 20. The prior practice in box annealin in ferrousenameling stocks confined such annealing substantially to the crosshatohed zone marked 2|; while normalizing of such enameling sheet stocks took place in the crosshatched zone marked 22. But operations within the critical range between the A1 and A: limits for a given composttion of, ferrous enameling sheet or strip have heretofore been considered deleterious andimpossible for any satisfactory result.

The present invention is depicted diagram matically in the phase diagram of Figure 6 in which the line of phase change, corresponding to line Na in Figure 5, is indicated at 23 and the A3 limit at 24 with the A2 line being designated by the numeral 25. A vertical line 26 represents atypical composition ferrous enamelingsheet or "strip steel containing ,about' 0.025% carbon. :l lnder this invention, the box annealing of such strip in' loosely coiled form in a suitable box an- Such sheets nealing furnace such as that'illustrated in Figures 1 and 2 iscarried on at'a temperatu're within the critical temperature range of such steel for a period of time at;temperaturefcommonly called thesoakin'g period, necessary toobtairi a suitable; soft; ductile quality and substantially uniform grain structurebefore cooling is permitted to-begin. Such box annealing between line 23 or the A1 line and the A3 points not only produces'a sheet having good deep-drawing and mechanical properties but it also conditions the carbide formation and renders it sufiiciently stable so that in the subsequent vitreous enameling firing opera tion mirror design blisters and copperheads do not occur to any commerciallydetrimental extent. Carb'onrosettes', including grain size, are illustrated in Figure 7 at anapproximate'mag'nification of 1000 times, using an etchant called nital (ethyl or methyl alcohol and nitric acid) to bring out the thin dark lines which are-the grain boundaries and the large black areas which are the pearlitic patches called rosettes. Such rosettes as may be formed by the agglomeration of the carbon or carbide elements in the enameling sheet stock by raising the temperature above the lower critical or A1 limit, when etched speci'fically for carbides with sodium picrate and at the same approximate magnification have the'general appearance illustrated in Figure 8. When, however, ferrous vitreous enameling sheet stock, is processed in accordance with this invention, suchcarbides are relatively dispersed and are stabilized and take the form as depicted by the illustration in Figure 9, using an etchant such as sodium picrate at approximately the same magnification. The carbide structure of Figure 9 does not give rise to the gas evolution difliculties in the vitreous enamel firing step which are occasioned when the carbides are in the form shown in Figures '7 and 8. 1 A

Some latitude is permissible in the; time and temperature relationship which in a given case employing the instant invention would permit of some change in time element for a corresponding and inverse change in temperature element. By way of illustration of the time-temperature relationship, sheets treated in accordance with this invention exhibited a Rockwell B hardness between 38.5 and 39.0 with a grain size number of 6 to '7 when the respective sheetswere treated at 1500F. for 4 hoursjat 1550 F. for 2 hours and at 1625 F. for 1 hour. In one suitable so-called austenitic grain-size chart commonly used for the classification of grains, the following numbers are given to the designated range of grain sizes measured in grains per square inch at magnifications:

N o. of Grains Gram Size No. per Sq. In. at

Similarly, some latitude exists in the heat input rates, the cooling rates, and the components of the stock as to which in order to obtain one or more special effects some other factor or factors may be slightly vcompromised without departing from the scope of this invention. 1

Further, the practice of this invention yields a ferrous enameling sheet in which the grain strucsnow? sheets, ;are;permitted tq exceed the A rtemperature limit for any-prolonged period of timebecauset-henthe grain structure in those top-sheets would tend to grow to objectionable size. for drawing (grain size numbers up :.to-about 4) and might insteadof being uniform be a combination of verylarge sizesurfacefgrains and relativelysmall size grains at the center of the gage. ,J

- In the preferred practicefof this invention, such boxennealing of a typical simple carbon steel enameling .stock takes place within the temperaturerange from about 1500" F. to about 1600 F. with a suitablesoaking; time, and 510w, conven-, tional cooling. This soaking time may vary from the orderof from about} .to about 10.:hours and is a material reductionin timeover. prior practice and hencematerially boosts capacity achieved under this invention onstill anothercount. The

charge in the furnace. inight bebroughtfu-D. to

annealing, temperature; within about 30. to gao hours, soak and be cooled outloftheairand under covertoabout 30.0? F.- (usually atfa rateno'tj, in excess o fa maximum. of 20.0." F1 per hour) before being cooled in theopenthe rest of theway down to atmospheric. temperature. .-Where. ver-y fine temperature control can be obtained,'the hottest portion of the sheets or strip. being. so box annealed should be maintained as much on the high side in the critical range and justbelow the A3 point as possible for a, give composition." In gen.- eral, it has been. observed that. by means of a 7 single, box annealing operation ferrous vitreous enameling sheets treated in accordance with this invention may be. obtained. with a Rockwell, B hardness; between 33- and- 38. approximately, with yield points varying between 21,000 and 26,000 pounds per square inch approximately;v with an ultimate strength. varying between 40,000 and 42,800 pounds per square inchapproximately; with a'percehtageelongation in 2 varying between 36.5 'and'lll' approximately; and with .a grain sizeabout 5. to 6 approximately, Whenannealed atabout 1600 F. for about l hours. These figures are but an indication of the highly usefulandunexpectedly satisfactory results'which are obtainable by the practice of this. invention.

.In the following representative comparison ferrous enamelin sheet stock typical of the type considered herein will under one practice of this invention and under conventional normalizing give the following estimated results, although this invention is not to be limited to the specific figures shown:

Annealing by Present. Conventional Invention Normalizing Between Above A; A1 andAa Point .Points Bottom temperature (of material), F l, 525 1, 700 Top temperature f material), E... l, 600 l, 700 Time to bring to temperature l; 30 hours 4 minutes Soaking pcriod 3 hours 2 minutes Cooling time before reaching air 12 hours 12 minutes Yield point, lbs./in. 26, 000-32, 000 Ultimate. s r /i 40, 000-43, 000 44, 000-46, 000 Per centElongationin 2 36 40 33-37 Olsen Ductility; 360-. 430 370-. 420 Rockwell B Hardness 33-38 40-47 GrainSizc N o. -6 5-6 Itwill, be noted, the grain. size obtained in. the practice otthis invention is uniform and sub:- stantially no largergthan the grains producedby normalizing, Although the annealed steel has a yield ,point which is somewhat lower. than the normalizedl product, any such lowering'off ers'f'a greater spread in theplastic range ,be fjorejfailure and therefore contributes to the drawing qualities of the metal, Furthermore, the slow coolingoi the box annealing in accordance with this finYention avoids the hardening and strain risk attend.-

1 ant upon the use of the. more expensive GQnYQnf tional normalizing without obj ectionable grain growth. I v g L In one annealing operation of a coil chargeoi tons in accordance with this invention. it 76- quired 41 hours for the charge to reach a metal temperature of l575l.F;-16 25 F., at which temperature the chargewas held for a period oi about 4 hours. The gage of the metal in the coils varied from .0359 to .0478". After anneal.- ing, the grain size numbers of the. charge ranged from.5 to 7.; the Rockwell B hardnessjranged between 31 and 41 and averaged 37; the yield strength of the charge in pounds per square inch ranged fromv 21,000 to 28100; the ultimate strength, from 40,000 .pounds 70 45,800 pounds per square inch; and the percentage elongation in 2". ranged fromf32 to 41; These resultsare but one example of the advantages of this invention. r

Although I have illustrated and described a preferred practice and embodiment of the invention, it will be understood that changes may be made therein without departing from the spirit of the invention or the scope of the appended claims.

Iclaimz 1. A process, for imparting ductility and carbide stability to very low carbon ferrous vitreous enameling sheet stock aft-er completion of the reduction thereof to an ordered thickness, comprising box annealing said sheet stock in loosely coiled strip formbetween about 1500 F. and about 1600 F. for a period of timesufficient substantiallyto stabilize the carbides therein, and then relatively slowly cooling said stock.

2. A process for producing a finally annealed lowmetalloid ferrous vitreous enameling sheet stock, comprising cold reducing a strip of enameling steel to an ordered thickness of said stock. releasing the coiling tensiongof. said, cold reduced stock, loosely recoiling said, stock, arranging said loosely coiled strip in a. box annealing furnace on edge, annealing said' loosely coiled stock in said furnace between about 1500 F. and about 1600 F. for a soaking period of time sumcient to stabilize the carbides and varying between about 2 hours and about 10 hours, slowly cooling said annealed stock substantially to atmospheric temperature. and, maintaining a nonscaling atmosphere about said loosely coiled stock, during sai annealing and said slow cooling until. said loosely coiled stock has reached a. temperature of about 300 F., whereby commercially practicable vitreous enameling sheets are produced having stable carbides, amaximumRockwell B hardness of 40, a minimum percentage elongation in .2. of about 32, aminimumyield point of about 22,000 pounds per square inch, a minimum. ultimate strength of about 38,000 pounds per square. inch, and a maximum grain size number of .about 5.

3. In a method of making enameling sheets, the steps including cold-rolling low-carbon fer..- rous strip suitable ior vitreous enameling subs stantially to final gauge, coiling the strip leaving the turns of the coils loose, box annealing the resulting coils by heating them in a non-oxidizing atmosphere to a temperature between 1550 and 1650 F., then cooling them in said atmosphere to about 300 F. thereby eliminating the tendency of sheets cut from the strip to exhibit mirror blisters on vitreous enameling.

4. In a method of making enameling sheets, the steps including rolling low-carbon ferrous strip suitable for vitreous enameling substantially to final gauge, coiling the strip leaving the turns of the coils loose, box annealing the resulting coils by heating them in a non-scaling atmosphere to a temperature between about 1550 F. and 1650 F., then cooling them in said atmosphere thereby preparing them for vitreous enameling.

JOSEPH C. ECKEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Metals Handbook, 1948 ed., p. 354, published 15 by American Society for Metals.

The Metallurgy of Deep Drawing and Pressing, by Jevons,pp. 199-200, John Wiley & Son, 1942. ,..=M. rm1es:i%1l 

1. A PROCESS FOR IMPARTING DUCTILITY AND CARBIDE STABILITY TO VERY LOW CARBON FERROUS VITREOUS ENAMELING SHEET STOCK AFTER COMPLETION OF THE REDUCTION THEREOF TO AN ORDERED THICKNESS, COMPRISING BOX ANNEALING SAID SHEET STOCK IN LOOSELY COILED STRIP FORM BETWEEN ABOUT 1500* F. AND ABOUT 1600* F. FOR A PERIOD OF TIME SUFFICIENT SUBSTANTIALLY TO STABILIZE THE CARBIDES THEREIN, AND THEN RELATIVELY SLOWLY COOLING SAID STOCK. 